Syntheses of many organic pigments include a coupling step in a dilute aqueous medium to produce a slurry of the pigment product, which is typically followed by filtering the slurry in a filter press to concentrate the pigment. The press cake that results is then either dried to provide a dry, particulate pigment or else is "flushed" with an organic medium such as an oil and/or resin to transfer the pigment particles from the aqueous press cake to the oil or resin phase. Flushing assists in keeping pigment particles non-agglomerated and easier to use in making inks or coatings. The flushing process requires additional time and materials over simply drying the pigment. If the pigment is used in an ink or coating composition, however, it must first be well-dispersed in an appropriate organic medium in order to achieve the desired color development and stability, and thus the flushing process is advantageous because it accomplishes the transfer without intermediate steps of drying the pigment and grinding the pigment in the organic medium to produce the pigment dispersion.
In the past, pigment flushes have usually been prepared by batch processes in which the press cake is kneaded with an organic phase such as an oil and/or a resin, for example in a sigma blade mixer or dough mixer, to flush the pigment particles from the water phase to the organic medium phase and displace the water as a separate aqueous phase. The displaced water is separated and the dispersion of the pigment in the varnish can be used as a pigment paste in preparing an ink or paint.
The batch process has many shortcomings. First, the steps of adding varnish, kneading the dough to displace the water, and pouring off the water must usually be repeated a number of times in order to obtain the optimum yield and a product with the desired low water content. This is a labor-intensive process that requires careful monitoring. Further, in order to remove the residual water, the batch must be further treated, such as by heating and stripping under vacuum. For many pigments, the heat history from processing to remove the residual water may result in a color shift. Further, the process is time-consuming and inefficient. Finally, it is difficult to reduce the water content below about 3% by weight, even with the vacuum stripping.
Continuous flush processes have been suggested in the past, but those processes have also had shortcomings. Higuchi et al., U.S. Pat. No. 4,474,473, describe a process for continuously flushing pigment press cake on equipment that includes a co-rotating, twin screw extruder. The process requires a press cake that has a pigment content of 35 weight percent or more. The '473 patent discloses that press cakes having a pigment content of from 15 to 35 weight percent cannot be used in the continuous process because of problems with obtaining constant flow feeding. The range of 15 to 35 weight percent, however, is the range of pigment content that is typically obtained for press cakes. While dilution of the press cake with water to form a liquid slurry of low pigment content was previously suggested, the '473 patent takes the opposite direction of increasing pigment content to 35% or more to provide a "lump cake" that is apparently suitable for constant flow feeding as a free-flowing solid. Increasing the pigment content of the manufactured press cake, however, requires a time-consuming process of shaping the press cake and drying it with circulating air until the desired water content is obtained.
An example of the methods using diluted press cake is Rouwhorst et al., U.S. Pat. No. 4,309,223. This patent discloses a process of preparing a pigment flush from a press cake using a single screw extruder. The process uses a slurry containing only about 0.5% to 10% by weight pigment. When so much water is added during the flushing process it is difficult to get a clean break or separation between the phases. In addition, more aqueous waste is produced. Finally, it is often the case that the single screw extruder does not provide a sufficient amount of mixing shear to adequately flush the press cake.
Anderson et al., U.S. Pat. No. 5,151,026, discloses an extruder apparatus for removing liquid from an aqueous mass of comminuted solids such as crumb rubber, wood pulp, and ground plastic materials that are cleansed during recycling processes. The water is squeezed out of the aqueous mass in a pinch point. The pinch point pressure results from applying a backward force by means of a reverse-threaded section of the screw immediately at the liquid extraction location. The Anderson process removes from water relatively large solid pieces that do not appear to associate or agglomerate. Unlike the Anderson process, the pigment flush process concerns transfer of fine pigment particles from aqueous press cake to an organic phase, usually including a resin, followed by separation of the two liquid phases (aqueous and organic). Two key considerations in the flush process are clean separation of the organic and aqueous phases and good dispersion of the pigment particles. The pinch point method is unsuitable for the two-phase pigment flushing process because the pinching force would interfere with the necessary phase separation between aqueous and organic phases. The pigment particles also have a tendency to agglomerate. The pinch point would thus be unsuitable for the additional reason that squeezing the pigment would cause undesirable agglomeration of the pigment particles, which would in turn impair dispersion of the pigment.